This application is related to, and being filed concurrently with, an application by Daniel J. Sherlock et al., entitled xe2x80x9cPosition Detection of Deployable Display Using Optical Encoder.xe2x80x9d
1. Field of the Invention
The invention relates generally to an obstruction detection system and method and, more particularly, to a system and method of detecting obstructions in the deploy path of a retractable liquid crystal display (LCD) monitor.
2. Description of the Related Art
A rectractable LCD monitor 10, illustrated in FIG. 1, is employed in many in-flight entertainment (IFE) systems. It is typically mounted underneath an overhead compartment area 20 and has two standard positionsxe2x80x94the stowed (inoperative) position 31 and the deployed (operative) position 32. While in the stowed position, the monitor is received in a recess provided underneath the overhead compartment area 20 and is held in that position against gravity by a latch 21. The deployed position of the monitor extends slightly past the vertical axis 41 to provide a more comfortable viewing angle to the passengers, which is indicated by a set of arrows labeled 42. While in the deployed position, an electromechanical brake (not shown) is typically used to hold the monitor in that position against a rubber bumper 22.
As shown in FIG. 2, the monitor is moved from its stowed position to its deployed position by a motor shaft 14 that is rotatably driven by a motor 15. The motor 15 is under the control of a motor control circuit 16. The motor control circuit 16 commands the motor 15 to rotate the motor shaft 14 in the forward direction (FWD) so as to move the monitor from its stowed position to its deployed position at a prescribed deploy velocity xcfx89. The deploy velocity has a prescribed upper limit which is equal to a rotational velocity of the monitor that would impart a maximum force, typically with a value of about of 6.5 pounds. Further, if an obstruction is present in the path of the monitor while the monitor is being moved to its deployed position, i.e. during the deploy path, so that the monitor encounters a force of typically 4.5 to 6.5 pounds, the monitor is required at that time to return to its stowed position, i.e., retract, and re-attempt deployment thereafter. Monitor deployment is attempted typically three times and if still unsuccessful after three attempts, the monitor remains in the stowed position until the unit is commanded on again.
In the conventional system, the obstruction is assumed to be present in the path of the monitor in one of two ways. In the first approach, the current through the motor is sensed and if there is an unacceptable increase in the current through the motor, the monitor is returned to its stowed position and monitor deployment is re-attempted. However, this approach is difficult to achieve consistently with temperature changes and over the life of the unit due to changes in the internal friction in the drive mechanism. Further, this approach often requires a separate calibration of the threshold current for each monitor on an aircraft because the current requirements for the motor to move the monitor at the prescribed deploy velocity are likely to be different for each monitor as the internal friction in the drive mechanism invariably differs from unit to unit.
In the second approach, the monitor is allotted a fixed period of time to deploy. If it fails to reach the deployed position within the allotted time, the monitor is returned to its stowed position and monitor deployment is re-attempted. However, with this approach, the monitor will continually press against the obstacle that is keeping it from deploying until the allotted time expires. This is undesirable especially when the obstacle is the passenger""s head.
An object of the invention is to provide an obstruction detection system and method for a retractable LCD monitor, which exhibits a greater degree of repeatability in setting the monitor to retract when encountering an obstruction.
The above and other objects of the invention are achieved with an optical sensor or switch that generate pulses indicating the speed at which the monitor is moving toward the deployed position and a retriggerable monostable multivibrator (also called xe2x80x9ca retriggerable one-shotxe2x80x9d). The retriggerable one-shot receives the pulses and when the pulses fail to arrive fast enough, i.e., below a threshold rate, the retriggerable one-shot times out and its output state transitions from a high level to a low level. The high-to-low state transition causes the monitor to retract and re-attempt deployment.
The invention provides greater repeatability because the variable that is sensed and monitored over time is the deploy speed of the monitor. Based on this data and a time reference, the force applied by an obstruction in the deploy path can be derived and when this force exceeds a certain value, the monitor is commanded to retract. As a result, the conditions under which retraction should occur can be determined from the deploy speed data of the monitor, irregardless of the other variables that the conventional system needed to take into account, in particular the surrounding temperature and the internal friction of the drive mechanism. Further, the invention employs digital circuitry which is more stable over life and environmental changes than analog circuitry that is used to sense motor current. As a result, the invention provides the potential for an adjustment-free design, and reduces the need to readjust units in the field.
Additional objects, features and advantages of the invention will be set forth in the description of preferred embodiments which follows.